Statistics say it's not important. Few people actually want to play older games on new systems. Enthusiasts like people who frequent gaming forums aside, the general population of gamers almost never go back to play games from previous generations, even if their system is backwards compatible. That's the simple truth of the matter. Many of the people who say it's important from a philosophical standpoint would probably never use it to a substantial degree.
Of course there are people who do like to play older games, and I do agree that it's a nice selling point at the start of a generation when a system has few games. It's unfortunate in those instances but sometimes abandoning that functionality is the only thing that makes sense.
Personally, I don't find it to be much of a consideration. My PS2 is hooked up right now but I probably haven't turned it on in a few years. I don't really replay older games much (other than FFT), as I have a backlog of new games to play.
All that being said, I really do think Sony will get something working with regards to PSN games. It might be slow, but I think something will be worked out for many titles. Still, as long as PSN works on the PS3, I intend to keep that console even after I get a PS4, so I'm golden either way.
